A number of diagnostic assays have been developed which rely on detection of the presence of a particular DNA or RNA sequence as an indication of the presence of an analyte, e.g., a bacterium, virus, or genetic defect, in a sample. In some cases the diagnostic gene is present in sufficient quantities to be detected directly, whether by hybridization, reaction with a specific antibody, or by some other method. However, if the gene of interest is present in a small amount or the background caused by similar sequences present in the sample is sufficiently high, reliable and sensitive detection of the targetted gene is difficult. An ambiguous result is not satisfactory in a diagnostic test.
Various techniques for increasing the sensitivity and specificity of such diagnostic procedures have been developed. Amplification of the target by cell culture, an efficient but time consuming technique, has been for long the only reliable method. Other techniques increase sensitivity of the detection system using sensitive reporter groups attatched to the probe which will combine with the target. Examples of sensitive reporter groups would include radioactive and fluorescent molecules. Enzymes, such as peroxidase or alkaline phosphatase coupled to the probe, also improve sensitivity through their catalytic action on substrate chromophores. Increased sensitivity may also be obtained by an amplification of the reporter groups. Such amplification has been achieved through avidin-biotin interactions, networking with nucleic acids, or the direct enzymatic replication of an RNA reporter group. This latter technique generates up to 1,000,000 copies of the RNA in about 12 minutes. Another technique amplifies the target nucleic acid sequence rather than the reporter groups used in the detection system.
One method for amplification of target nucleic acid is known as the polymerase chain reaction or PCR technique and has been developed for detecting the genes responsible for genetic defects. This method uses specific oligonucleotide primers in repeated cycles of target DNA denaturation, primer annealing, and extension with a DNA polymerase. Extension products generated from one primer serve as additional target sequences for the other primer. The degree of amplification of a target sequence is controlled by the number of cycles that are performed and is theoretically calculated by the simple formula 2.sup.n where n is the number of cycles. Given that the average efficiency per cycle ranges from about 65% to 85%, 25 cycles are needed to yield from 0.3 to 4.8 million copies of the target sequence.
Although the polymerase chain reaction is a very sensitive and promising method, there are some limitations and disadvantages inherent in this technique. For example, each cycle of polymerase chain reaction provides at best only a 2-fold amplification, and thus a high number of cycles (between 20 and 30) is required to achieve substantial amplification. Furthermore, the high-temperature denaturation that occurs in each PCR cycle typically inactivates the enzyme used and thus requires repeated addition of expensive enzyme.
Accordingly, techniques that increase the rate of gene amplification (thereby requiring less enzyme and fewer cycles) would be highly advantageous to all diagnostic techniques that involve the detection of a specific target nucleotide sequence and any other procedure needing an increased number of specifically amplified polynucleotides (RNA or DNA).